1. Field of the Invention
The present invention relates to an image inputting apparatus such as a camera device equipped with an AF device by a so-called contrast AF method.
2. Description of the Prior Art
Conventionally, an image inputting apparatus such as a still camera and a video camera is equipped with an AF (Auto Focusing) device for optimally adjusting a focusing condition of an image of a subject automatically.
An auto focusing device in a traditional image inputting apparatus that records the image of the subject to a recording medium such as a film and a video tape and the like as an image and a vision, is provided with a ranging device for obtaining a distance to the subject, and employs an AF which controls a focus driving device to move a lens to a focal position which corresponds to the distance obtained by the ranging device. Such AF is referred to as an outside light AF in contrast with a contrast AF, which will be described later.
The outside light AF is classified broadly into a passive AF and an active AF depending on a ranging method performed by the ranging device. In the passive AF, ranging is carried out based on a phase difference of a plurality of images of the subject projected on the ranging device by passing through an optical path which is different from an optical path passing trough a photographic optical system. In the active AF, for example a near-infrared ray or a supersonic wave or the like is radiated to the subject and ranging is carried out based on a time or an angle of a reflected wave from the subject to be returned to the ranging device.
Meanwhile, although there is a case in the active AF that the distance to the subject cannot be detected accurately since, for example, in a case of photographing the subject passing through a windowpane, the near-infrared ray and the like radiated to the subject is reflected by the windowpane, the passive AF is difficult to be influenced even by such case that the subject is pass through the windowpane. Therefore, there is a tendency that the passive AF is employed for an image inputting apparatus which accuracy is relatively required.
On the other hand, in a new image inputting apparatus such as a digital camera and the like which projects the subject image on an imaging device such as a CCD through the photographic optical system and retrieves the projected image of the subject as an electric signal, because an image signal of the subject image can be retrieved virtually simultaneously with the projection of the subject image onto the CCD, it is possible to perform an AF operation by utilizing this image signal.
More specifically, this AF moves a focusing lens of the photographic optical system, and evaluates contrast (such as sharpness of image) of the image projected on the imaging device in real-time based on the image signal at each time of the movement of the focusing lens while the focusing lens is moved, and stops the movement of the photographic optical system at a position where a value of this evaluation becomes greatest. This AF is referred to as the contrast AF, and is also referred to as a CCDAF in which the CCD is used as the imaging device.
The contrast AF has a characteristic that a focusing accuracy is extremely high, since the contrast AF is carried out by evaluating the image which is projected actually onto the imaging device rather than obtaining the distance to the subject.
In addition, since the contrast AF is carried out by comparing the evaluation values in each moving position of the focusing lens while moving the focusing lens, it is possible to improve the focusing accuracy if more repeated numbers of a series of operations which include accumulation of the images, transferring thereof, computation of the evaluation values and comparing of the computed values, are carried out.
On the other hand, there is a problem that a time required for a determination of a focusing position (completion of the AF operation) becomes long, if the many operations are repeated. Under the circumstance that the time required from the beginning of the AF operation to the completion of the AF operation, namely, a time-lag influences significantly on a focusing operation, and accordingly, in a case of photographing the subject which is moved at high speed for example, there may be a case that the photographing cannot be performed with a desired timing.
Also, in the contrast AF, a significant difference does not occur in the evaluation value in a dark circumstance where the entire contrast is low, and thus there is even a case that the appropriate focusing operation cannot be performed.
On the contrary, although the outside light AF is inferior to the contrast AF in terms of the focusing accuracy, the outside light AF can follow well to a dynamic subject since the time-lag required for the AF operation is less, therefore, loss of timing for the photographing can be avoided.
In addition, the outside light AF has a characteristic that the outside light is generally difficult to be influenced by the contrast of the subject.
Therefore, there is proposed in Japanese Patent Laid Open No. 2001-255456 a hybrid AF (HBAF) provided with the AF device for performing an operation of the outside light AF and the AF device for performing an operation of the contrast AF as mentioned above, and capable of alternatively changing over both the outside light AF and the contrast AF operations according to a photographing situation and the like.
In addition, the Applicant of the present application has also proposed in unpublished Japanese Patent Application Nos. 2002-175297 and 2002-231065 a HBAF capable of changing over to the contrast AF under a predetermined condition while based on the outside light AF within a range of a normal photographing operation.
By the way, a range which a certain resolution can be obtained is judged as an allowable range of focusing in the image inputting apparatus, and the range which the certain resolution can be obtained is decided by a width of a difference between a CCD surface and an actual imaging surface. More specifically, the image inputting apparatus controls the different width between the CCD surface and the actual imaging surface so as to obtain the resolution. In practice, the image inputting apparatus is adapted to adjust the different width between the CCD surface and the actual imaging surface by moving the focusing lens.
FIGS. 6 and 7 are diagrams to explain the control of the different width between the CCD surface and the actual imaging surface in this image inputting apparatus. FIG. 6 shows a case in which the subject is in a long distance from the image inputting apparatus, and FIG. 7 shows a case in which the subject is in a short distance from the image inputting apparatus. Here, since a depth of image plane (an allowable range of the difference between the CCD surface and the actual imaging surface) is same in both cases that the subject is in the long distance and the subject is in the short distance from the image inputting apparatus, and a width of movement of the focusing lens that corresponds to the adjustment of the difference width between the CCD surface and the actual imaging surface in the both cases is also virtually the same in both cases that the subject is in the long distance and the subject is in the short distance from the image inputting apparatus, a relation of X1≈X2 is established between a width of movement X1 of the focusing lens in the case of the long distance and a width of movement X2 of the focusing lens in the case of the short distance.
Therefore, it can be said that the focusing accuracy does not change in the case of the long distance and even in the case of the short distance, theoretically.
However, a distance from a position of the subject to a focused position when the different width between the CCD surface and the actual imaging surface is the same differs greatly between the case of the long distance and the case of the short distance. More specifically, as shown in a middle drawing of FIG. 6, in the case of the long distance, the distance from the position of the subject to the focused position is Δ1 when the different width between the CCD surface and the actual imaging surface is δ1. On the other hand, in the case of the short distance, as shown in middle diagram of FIG. 7, the distance from the position of the subject to the focused position is Δ2 when the different width between the CCD surface and the actual imaging surface is δ2 (=δ1). Therefore, a relation of Δ1 >>>Δ2 is established. More specifically, it can be said that a depth of field to a certain depth of image plane is deeper in the case of the long distance compared with the case of the short distance.
In a concrete example, when the photographing is performed by a telescopic lens, although in the case of the long distance, the depth of field corresponding to a certain depth of image plane is, for example 16 m−30 m and thus there is a width of 14 m, in the case of the short distance, the depth of field is, for example 0.6 m−0.613 m and thus there is a width of only 0.013 m. This means that, for example, a situation that a tree quite in front of a subject becomes in focus when a building in 30 m ahead is set as the subject, and a situation that bangs of a person in 0.613 m ahead become in focus rather than eyes of that person becomes in focus when the person in 0.613 m ahead is set as the subject, occur by the same difference width between the CCD surface and the actual image plane.
In these situations, it can be said that the width of movement of the focusing lens is virtually the same as long as the different width between the CCD surface and the actual imaging surface is the same, thus the focusing accuracy between the case of the long distance and the case of the short distance is recognized as the same in the image inputting apparatus.
However, these situations are difficult to be recognized as the same focusing accuracy from sensitivity of a user viewing actually photographed photos. More specifically, under the same focusing accuracy, although it is merely recognized by the user that the case of the short distance is slightly out of focus by a sensation, there is a problem that the case of the long distance is recognized as evidently out of focus.
In addition, in a case that a subject is mixed with the long distance and the short distance from the image inputting apparatus, in terms of a fact that it is difficult to avoid the focusing to be more or less out of focus since judging primarily where to focus optimally in such case is difficult, it can be said that a range of being recognized as in focus by the sensation of the user is broad compared with the subject in a certain distance (single distance).
As stated above, the sensation that the user actually feels varies among the case that the subject is in the short distance and the case that the subject is in the long distance from the image inputting apparatus, and furthermore the case that the subject is in the certain distance and the case that the subject is mixed with the long and the short distances from the image inputting apparatus, even the focusing accuracy is the same.